In transflective liquid crystal displays (LCDs), quarter wave film (kind of phase difference film)s impart a phase difference of ¼ wavelength to linearly polarized light passing through a polarization film, thereby converting the linearly polarized light into circularly polarized light.
FIG. 1 is a cross-sectional view schematically illustrating the structure of a laminate constituting a transflective LCD. In FIG. 1, one section associated with indispensable elements of the present invention is enlarged for a better understanding of the present invention. In actual LCDs, an additional layer may be arranged, if necessary.
LCDs transmit information by blocking or transmitting light emitted from a backlight 7 based on polarization until the light is visible to the naked eye through a variety of paths. Generally, light emitted from backlight 7 is visible to the naked eye through a variety of paths, as shown in FIG. 1. Accordingly, the light emitted from backlight 7 can be partially seen to the naked eye.
Based on this point, there have been developed transflective LCDs. More specifically, while utilizing light commonly used in backlights, transflective LCDs reflect light in a region where a separate light source is provided, thereby exhibiting an improved brightness.
The transflective LCDs include the quarter wave films 2 and 5. The reason will be described with reference to FIGS. 1 and 2. In FIG. 2, a portion represented by “x” means that light is emitted to the inside of the LCD, and a portion represented by “•” means that light is emitted to the outside of the LCD.
First, a detailed description will be made with regard to a transflective LCD where an electric field is applied to a liquid crystal layer 3. Light supplied from an external light source, rather than a backlight, is linearly polarized while passing through an uppermost external polarization film 1 of the LCD (SN1). After passing through the quarter wave film 2, the linearly polarized light undergoes the phase difference of a ¼ wavelength, thus being converted into circularly polarized light (SN2). The circularly polarized light transmits the liquid crystal layer 3, to which an electric field is applied. As a result, the liquid crystal loses its original orientation, thus the orientation is changed. That is, the liquid crystal has no specific orientation causing phase difference, thus undergoing no a change in polarization state (SN3). The circularly polarized light is reflected by a reflection plate 4 (SN4). The reflected circularly polarized light passes through the liquid crystal layer 3 while maintaining the same phase difference. As mentioned above, the liquid crystal layer 3 has no specific orientation to create a phase difference, thus undergoing no a change in polarization state (SN5). The circularly polarized light retransmits the quarter wave film 2. At this time, the phase difference of a ¼ wavelength occurs. By adding this phase difference to the phase difference of a ¼ wavelength occurred in SN2, there occurs a total phase difference of a ½ wavelength. The light is finally converted into linearly polarized light equivalent to a 90 degrees rotation from the polarization direction of the linearly polarized light firstly transmitted to the polarization film 1 (SN6). As a result, the polarization direction of light which reaches the uppermost external polarization film 1 is perpendicular to that of the polarization film 1. The light fails to transmit the polarization film 1, thus being blocked by the polarization film 1. Accordingly, in a transflective LCD, where an electric field is applied to a liquid crystal layer 3, light supplied from the external source undergoes no reflection.
On the other hand, in a case where a liquid crystal layer 3 maintains its original orientation without any application of an electric field, the opposite result will be obtained as follows. Light supplied from an external light source is linearly polarized after passing through the polarization film 1 of the LCD (SY1). After passing through the quarter wave film 2, the linearly polarized light is converted into circularly polarized light (SY2). SN1 and SN2 are the same as described above. However, in transmission of the circularly polarized light into the liquid crystal layer 3, there be obtained results different as described above. More specifically, the crystal liquid layer 3 is free from an electric field, thus maintaining its original orientation due to the interaction with the orientation film. Accordingly, the phase difference of a ¼ wavelength can be obtained by controlling the thickness of the crystal liquid layer 3. For this reason, the circularly polarized light in SY2 undergoes a ¼ wavelength the phase difference, thus being perpendicularly polarized to the linearly polarized light in SY1 (SY3). The linearly polarized light is reflected by a reflection plate 4 (SY4). The reflected linearly polarized light passes through the liquid crystal layer 3 while maintaining the same phase difference without any variation in polarization state.
Similarly, the liquid crystal layer 3 maintains its original orientation, thus undergoing the phase difference of a ¼ wavelength. As a result, the linearly polarized light is converted into linearly polarized light undergoing a total phase difference of a ¾ wavelength (SY5). Then, the linearly polarized light retransmits the quarter wave film 2. At this time, the phase difference of a ¼ wavelength occurs. By adding this phase difference to the phase difference of a ¾ wavelength in the previous step, there occurs a total phase difference of a 1 wavelength (i.e., a phase difference of zero). As a result, the polarization direction of light which reaches the uppermost external polarization film is the same as that of the polarization film, thus transmitting the polarization film and being visible to the naked eye. Accordingly, the transflective LCD, where no electric field is applied to a liquid crystal layer 3, has an advantage of improvement in brightness owing to the external source.
As apparent the foregoing, the quarter wave film is an essential element of the transflective LCD.
As shown in FIG. 1, in addition to the quarter wave film 2 arranged on the liquid crystal layer 3, another quarter wave film needs to be arranged at an opposite side of the film 2. It is because additional ¼ wavelength phase difference is needed to allow light emitted from the backlight and transmitting a glass substrate 7 and a lower polarization film 6 to transmit a upper phase polarization film 1 with undergoing no phase difference (i.e., 1 wavelength phase difference) in case that an electric field is applied to a liquid crystal layer. That is to say, a liquid crystal layer in a region, where there is no reflection plate, has a relatively large thickness, as compared to the case of a region where there is a reflective plate. By controlling the thickness of the crystal liquid layer, the phase difference of a ½ wavelength can be obtained upon formation of an electric field. The addition of this phase difference to the phase difference of a ¼ wavelength caused by the phase difference film 2 makes a total phase difference of a ¾ wavelength. The resulting light is converted into circularly polarized light having the phase difference of a ¾ wavelength. To convert the circularly polarized light into linearly polarized light equivalent to the phase difference of a 1 wavelength, there is a need for another quarter wave film 5 to induce an additional occurrence of a quarter wave.
As noted above, the transflective LCD requires in total two quarter wave films.
As shown in FIG. 3, no phase difference film creates the phase difference of a ¼ wavelength in response to light of all wavelengths. That is, the level of the phase difference is varied depending upon the wavelength of light. As can be seen from the curve of FIG. 3, generally, the longer wavelength, the smaller phase difference level. For this reason, in a wavelength range exception for a predetermined wavelength bandwidth, elliptically polarized light, rather than circularly polarized light is generated.
The elliptically polarized light makes it more difficult to control light transmittance based on polarization. To impart the phase difference of a ¼ wavelength to light of a wide wavelength bandwidth, the film essentially needs to be provided with achromaticity.
Generally, the occurrence of the phase difference of a ¼ wavelength for light of a wide wavelength bandwidth is obtained by laminating a ½ wavelength phase difference film and ¼ wavelength phase difference film such that the ½ wavelength phase difference film crosses the ¼ wavelength phase difference film at a specific angle.
Accordingly, the quarter wave film represented by reference numerals “2 or 5” in FIG. 1 is not a single layer. As shown in FIG. 4, the quarter wave film has typically a structure of triple-layer laminate consisting of the quarter wave film has typically a structure of triple-layer laminate consisting of a ¼ wavelength phase difference film 8 or 10 and a ½ wavelength film 10 or 8 which is laminated together and an adhesive layer 9 arranged between the two film to impart binding force therebetween.
For the ¼ and ½ wavelength phase difference films, there is generally used a film made of a polymer capable of exhibiting anisotropy via stretching in a specific direction, e.g., a cycloolefin polymer (COP) or polycarbonate (PC) polymer. The quarter wave film has the structure of a quarter wave film laminate 3 obtained by forming films having a predetermined thickness and laminating the films via the adhesive layer.
Since ¼ and ½ wavelength phase difference films are subjected to stretching to obtain anisotropy, they must be subjected to filmization prior to lamination. During the filmization, the phase difference films must have a sufficient thickness due to the filmization process. The minimal thickness of each film is about 40 μm. A total thickness the laminate consisting of the two films and the adhesive layer reaches about 100 μm.
Recent trends toward slimness of small-medium size display devices (e.g., cellular phones, PDAs and games) have been continued. The phase difference film laminate having a large thickness (about 100 μm) has been a great obstacle to the slimness of the display device.